Method of removing heavy metal ions from white fuming nitric acid



METHOD OF REMOVING HEAVY P/IETAL IONS FROM WHETE FUWNG NETREC ACE Howard M. Kindsvater and Don L. Armstrong, ovina, and Hides Watanabe, Pasadena, Calif, assignors to Aerojet-General Corporation, Azrrsa, Caiifi, a corporation of Ohio N Drawing. Application September 17, 1953,

Seriai No. 389,86t3

5 Claims. (Cl. 23159) This invention concerns the removal of metallic ions from white fuming nitric acid.

White fuming nitric acid, hereinafter referred to as WFNA, is often stored in stainless steel drums. During storage a certain amount of the drum is dissolved by the WFNA, and ions of iron, chromium and nickel are introduced into the acid. When the WFNA is used as an oxidizer for the fuel in a rocket motor, these ions may be deposited as metallic salts in the motor chamber. This frequently results in malfunction of the motor or complete motor failure due to clogging the injector nozzle or erosion of the turbine blades and other parts of the rocket motor.

Accordingly, it is an object of this invention to provide a way of reducing the concentration of metallic ions in WFNA.

A feature of the invention resides in the addition of concentrated phosphoric acid or anhydride to the WFNA, agitating the mixture, and filtering it. The salts, largely ferric phosphate, with some phosphates of the other metals named above which have been precipitated will thus be removed from the liquid.

This precipitate is assumed to be primarily composed of ferric phosphate and coprecipitated phosphoric acid. Ferric phosphate is Well known to be soluble in strong acids, such as nitric acid, see page 394 of Reference Book of Inorganic Chemistry, published 1940, however, we have found that ferric phosphate is relatively insoluble in WFNA. This might possibly be explained by the low dissociation constant of WFNA.

It has been found that a substantial excess over the stoichiometric amount of phosphoric acid or anhydride needed to react with the metal salts is required. A satisfactory excess is about 100% by weight. This excess apparently does not dilute the nitric acid for we have found that less than 0.1% of phosphoric acid remains in the WFNA. For best results the concentration of the phosphoric acid should be at least about 85% syrupy; and this is what is meant by concentrated phosphoric acid as used herein.

The quantity of phosphoric acid to be added is usually determined by preliminary analysis for metals of the WFNA to be purified.

The WFNA can be filtered within fifteen minutes after mixing with the phosphoric acid or anhydride, but a more complete separation of the metal ions can be accomplished if the acid is allowed to stand for several hours. Only a negligible amount of solid material remains in the WFNA after this treatment. The solids remaining behind probably represent the solubility of ferric phosphate in WFNA at equilibrium with the precipitate. In amounts they appear to be quite negligible, approximately 0.17 solids calculated as FezOs. However, flame photometer tests on the solid residue from treated acid do not confirm the presence of iron.

Very little phosphoric acid remains behind in the WFNA even though an excess of approximately 100% phosphoric acid is used. It has been found, as noted above, that less than 0.1% phosphoric acid remains in solution; most of the unreacted phosphoric acid being coprecipitated with the ferric phosphate. This amount has not been found to cause any difficulties in rocket motor operation.

This method of treatment is adaptable to large scale operation and has been successfully used in treatment of as much as 1000 gallons of WFNA in one operation, yielding substantially low concentrations of metallic ions as described above.

This invention is not limited except in accordance with the appended claims; and it is understood that when the term phosphoric acid is used in the claims, phosphoric anhydride is included in this expression.

We claim:

1. A method for removing heavy metal ions selected from the group consisting of iron, chromium, nickel and mixtures thereof from white fuming nitric acid which comprises adding concentrated phosphoric acid thereto in a quantity greater than the stoichiometric requirement for reaction with the dissolved metallic ions, and thereafter filtering the mixture.

2. A method for removing heavy metal ions selected from the group consisting of iron, chromium, nickel and mixtures thereof from White fuming nitric acid which comprises adding concentrated phosphoric acid thereto in a quantity at least equal to a 100 percent excess of phosphoric acid over the stoichiometric quantity required for complete reaction with the metallic ions, and thereafter filtering the mixture.

3. A method for removing heavy metal ions selected from the group consisting of iron, chromium, nickel and mixtures thereof from white fuming nitric acid which comprises adding concentrated phosphoric acid, syrupy, thereto in an amount greater than a percent excess over the stoichiometric requirements for a complete reaction with the metallic ions, and thereafter filtering the mixture.

4. A method for removing heavy metal ions selected from the group consisting of iron, chromium, nickel and mixtures thereof from white fuming nitric acid which comprises adding concentrated phosphoric acid, 85% syrupy, thereto in a quantity in at least a 100 percent excess over the stoichiometric requirement for a complete reaction with the metallic ions, agitating the mixture, permitting the mixture to stand, and thereafter filtering the mixture.

5. A method for removing metallic ions of iron,

chromium and nickel and mixtures thereof from white fuming nitric acid which comprises the addition of concentratecl phosphoric acid thereto, and thereafter filtering the mixture.

Hackhs Chemical Dictionary, 3rd ed., page 573; The Blakiston Co., Philadelphia, Pa. (1950). 

1. A METHOD FOR REMOVING HEAVY METAL IONS SELECTED FROM THE GROUP CONSISTING OF IRON, CHRONIUM, NICKEL AND MIXTURES THEREOF FROM WHITE FUMING NITRIC ACID WHICH COMPRISES ADDING CONCENTRATED PHOSPHORIC ACID THERETO IN A QUANTITY GREATER THAN THE STOICHIOMETRIC REQUIREMENT FOR REACTION WITH THE DISSOLVED METALLIC IONS, AND THEREAFTER FILTERING THE MIXTURE. 